Cognitive Neuroscience Lecture 6: Guest Lecture: Neuroprosthetics
L6: Guest Lecture—Neuroprosthetics Recreating sight in blind patients; case study Inspired by cochlear implant Four opportunities along visual pathway: Retina, optic nerve, LGN, Visual cortex Outer retinal degeneration: fewer retinal cells--Retinitis pigmentosa - So, we put a subretinal implant in the area of loss of rods and cones - Or, we put an epiretinal implant (easier for surgeon) Retinal implants started at wilmer - Small probe in the eye - Held by surgeon, stimulates retinal cells - Patient awake; local anesthesia New epiretinal approach: argus II retinal prosthesis system - Electrodes individually adjusted - 6x10 retinal implants - Res 525 Mm - Camera, transmitter coil, video processing unit, electronics case, receiver coil Many countries all over the world Implant approved in Europe: Alpha IMS - Electrodes jointly adjusted - Res 80 Mm Future: subretinal implant - Emphasis on extraocular transmission and processing away from stim site - Receiver coil and electronics on temporal sclera - Scleral tunnel to subretinal location Future: German Epi-Ret projects—epiretinal implants - External camera - Wireless transmission - Antenna and electronics in the capsular bag Stanford Optoelectronic Retinal Prosthesis Concept - Subretinal implant inducing retinal migration US DoE Artificial Retina project - Novel electrode materials and light capture - Many types of electrode arrays - Light-to energy conversion using plant photosynthetic molecule PS II and genetic engineering Neurotransmitter based retinal prosthesis (concept only, not real yet) - NT release targets inner retina - Specific pathway (on only or off only) excitation might be possible - Stim through microfluidic channels in chip - Testing in isolated retina prep Optic nerve prosthesis - Emphasis on processing crude info - Electrodes in cuff around optic nerve; light/dark/direction/stim strength must be learned - Penetrating electrodes being developed Concept of cortical prosthesis: - video encoder in glasses, electrode array in V1, transcranial interconnect in base of head, signal processor far below Intracortical approach: NINDS, 1992 - coil, ceramic, AIROF electrodes, counter + reference Cortical complication: Scrambled map - electrodes à retinal percept? - Cortical percept magnification isn’t exactly in order Near future: other active groups; human trials many years away Planning for prosthetics: Simulations - Where did SSMP get the idea for 16 and 60 electrodes - First systematic explorations through simulation - Principal fields of investigation: o Wayfinding/orientation & mobility o Visual acuity/reading o Face recognition o Hand/eye coordination (visually guided action) - Many stim are woefully simplistic (crisp round dots) or needlessly hard (eccentric pixelized reading) Route planning in real world - Pros o Easy to create o Useful for early trials - Cons o Layout quickly learned o Unnatural (why do that if cane/dog works just as well) o Potential risk Real-world wayfinding: results - Sighted subjects trained on pixelized viewing - Fast usually = sloppy - High res is better but not perfect - 6 x 10 good enough Hurdles for implant wearers: Stabilized view - Stabilized view o Needed because eye movement is a lot of extraneous movement o Subjects learn to hold eyes still o Subjects learn to move camera Modeling prosthetic vision in a nutshell - Filter software transforms camera image or virtual scene - Eye-tracking stabilizes raster location (optional) - Subject views scene in headset (monocularity) Eye-head-hand coordination Virtual mobility - Dots locked to eye position Hurdles for implant wearers: Spatial filtering - Prosthetic vision in the operating room - Stim single electrodes (retina or cortex): perceived in correct location, flicker perceived up to 40hz - Stim multiple electrodes (mostly retina): crude shape scene (single row/col), blurrier than dots, hard to see flicker - Retinal rewiring: inner nuclear layer cells migrate and form clusters - Meander mazes; maze tracing Hurdles for implant wearers: Temporal filtering - Maze tracing, temporal low-pass filter Hurdles for implant wearers: Spontaneous bg activity - All prostheses, even ideal ones, give light shows Argus II: moving shadows - prosthetic != normal Lots of training needed - Performs line task better than without it - Finding objects and seeing movement: Square localization/direction of motion - Spot reading and object recognition - Orientation and mobility